1.
Isohemagglutinins were demonstrated in the plasmas or sera of 24 out of 32 chickens which had been immunized with nonparasitized or parasitized chicken erythrocytes, but were evident in the plasma of only one out of 14 chickens which had recovered from P. lophurae infections induced with parasitized duck erythrocytes and had never been injected with chicken erythrocytes. Such antibodies were demonstrated also in plasma from one of seven normal chickens tested.

2.
Freezing and thawing markedly enhanced the agglutinating powers of 16 out of 18 isoagglutinating chicken plasmas tested and slightly enhanced the agglutinating power of one of the other two.

3.
Supplements of previously frozen plasmas from other chickens enhanced agglutination by unfrozen chicken isoagglutinating plasmas in 19 out of 22 tests. Supplements of unfrozen plasmas enhanced such agglutination in only eight of 23 tests. Frozen supplements, then, were more effective.

6.
Passive protection against P. lophurae was effected with plasmas from chickens which had been immunized against erythrocytes from other chickens, but had never been infected with malaria. Such plasmas were fully as effective as plasmas from chickens which had been immunized against both P. lophurae and erythrocytes from other chickens. The latter chickens were inoculated with chicken erythrocytes after recovering from P. lophurae infections induced with parasitized duck erythrocytes, or were infected and superinfected with P. lophurae by inoculation with parasitized chicken erythrocytes. Some protection was obtained with plasmas from chickens immunized against P. lophurae but not against chicken erythrocytes, but it was of a lower magnitude than that conferred by plasmas containing anti-erythrocytic antibodies.

7.
In one experiment, absorption with residues from lysed nonparasitized erythrocytes and with intact nonparasitized erythrocytes resulted in loss of protective power of plasma from chickens infected and superinfected with P. lophurae by inoculation with parasitized chicken erythrocytes. Chicks receiving the plasma absorbed with the residues from the lysed erythrocytes actually showed higher parasitemias during the first few days of the infection than did the control chicks. In a second experiment, absorption with residues from lysed nonparasitized erythrocytes did not alter the protective effect of a similar plasma.

8.
Passive protection was obtained when administration of plasma was begun two days after inoculation, as well as when plasma injections were begun prior to inoculation. No significant protection was evident when plasma was administered for only two days.

9.
Comparable protection was afforded by unfrozen and previously frozen plasmas from chickens immunized against chicken erythrocytes but not against malaria.